Schmidt television projector having meniscus to correct for antihalation lens



2,642,488 MENISCUS ENS J1me 1953 D. w. E-PSTEIN SCHMIDT TELEVISION PROJECTOR HA TO CORRECT R ANTIHALATI Fi Jan 3 194 VING 0N L 9 Patented June 16, 1953 SCHMIDT TELEVISION PROJECTOR HAVING MENISCUS TO CORRECT FOR ANTIHALA- TION LENS David W. Epstein, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 3, 1949, Serial N 0. 68,878.

4 Claims. (01. 340 -370) The present invention relates to image projec- /tion systems employing reflective optics, and more particularly, but not necessarily exclusively to an improved projection system of the type comprising a spherical mirror and a spherical aberration correcting means or correcting plate.

The present invention has for its principal aim to improve projection systems employing reflective optics by the addition of optical elements in a novel manner thereby to make possible the use of a weaker correcting plate. The Weaker correcting plate plus the added elements permits higher resolution for off-axis points.

Another and more specific aim or object of the invention is to provide an image projection system of the type employing a spherical mirror and correcting lens with a meniscus which is concentric with the spherical mirror thereby to permit use of a weaker correcting late. p A further object of the invention is to provide an image projection system of the type employing a spherical mirror and correcting lens with a meniscus and an antihalation lens which are arranged in a novel manner.

Other objects and advantages of the present invention will, of course, become apparent and immediately suggest themselves to those skilled in the art to which the invention is directed from a reading of the following specification in connection with the accompanying drawing in which:

Fig. 1 is a side elevational View, partly in section, of projection apparatus embodying the invention;

Fig. 2 is a view similar to Fig. 1 but showing projection apparatus embodying the invention in modified form;

Fig. 3 shows a variation in the arrangement of Fig. 2.

In the illustrative example, it is assumed that image signals are received from a suitable radio transmitter and are then amplified and demodulated in a suitable receiver. It is also assumed that the resulting image signal is supplied through a video amplifier (not shown) to appropriately modulate the electron beam developed within a cathode ray tube [0. With the tube in operation, the reproduced image appearing on the end thereof is projected upon an image viewing surface, such as that provided by a screen 14 by means of the improved optical system of the invention. The general organization of the projection system shown in the illustrative example comprises a member 16 having 2 a spherical reflecting surface I! and an aspheric zone plate I 8 (also termed a correcting plate or correcting lens). The correcting plate is located at the center of curvature C of the spherical reflecting surface H. An aperture or stop, such as that indicated at 26, located sub-- stantially at the center of curvature "of the spherical reflecting surface I! will, in general, improve the operation of the optical system.

A meniscus 2| has concentric spherical surfaces 22 and 23 and these surfaces are also concentric with the spherical mirror surface ll.

An antihalation lens 21 is in optical contact with the face of the tube l0. Optical details of an antihalation lens are disclosed and claimed in a copending application of the present applicant for Cathode Ray Tube Envelope, bearing Serial No. 17,967, filed March 30, 1948, now Patent No. 2,517,774, granted August 8, 1950. The following description is given for the sake of completeness of disclosure and is to be found in the application just referred to. The antihalation lens 21 is in the form of a concentric meniscus which is also concentric with the end of the tube I0. The thickness T of this lens 21 is given approximately by the relation:

in which R2 is the radius of curvature of the outer face of the lens 21. N is the index of refraction of the transparent material of the lens. The lens 21 may be in the form of a thick piece of molded transparent plastic in optical contact with the face of the tube l0. By way of example, the tube H1 or the end of the bulb 29 of the tube In may be of Pyrex glass. The optical member 21 may be made of methyl methacrylate in optical contact with the tube face insured by using an oil such as clear mineral oil of Canada balsam.

The focal length 1 of the spherical mirror, meniscus and correcting lens is given by is the power of the meniscus, and

The focal length F of the complete system is.

and is equal to the inner radius of curvature of the antihalation lens and thus the curvature of the focal surface (CR tube face). index of refraction of the lens 21.

If m be the magnification desired with this system, then the tube is located at the distance qi'from' the'center of curvature given as 1 HCYE) and the image will be located on a 'flatplane at Fig. 2 offthe drawings shows an embodiment of the invention which employs a plane mirror to reverse the direction. of the antihalation lens and the cathode ray tube. In Fig. 2 the meniscus 2|a .is substantially the same as the-meniscus 2! offFig. 1 of the drawings. This meniscus presents concentric spherical surfaces 22a and 23a. The surface 2 3a isflattened surrounding the axis of the system as shown by reference'character 5! which is silvered :or aluminized as indicated by reference character 42 to provide a plane 'mirror. A member 44 is provided with a spherical reflecting. surface 46 which is-concave'toW-ard the plane'mirror surface 32. The reflecting surface may be provided inany suitable Way-by suitable material. The center-of themirrorsurface 46 is indicated at C. Preferably the surfaces 22a and 230 are concentric with the surface 46 with their centers lying at C. The convex surface 48 of the member 44 has no optical significance and,

therefore, it may be-of any desired :or convenient configuration.- An antihalation lens: Si is slid able in. a bore 53 which is centeredin Lthe memre berMr The backsurface 56 of the membertti is provided with a spherical depression 58 .toreceive the-active surface-of a cathoderayjtubeilfim The back surface 56 surrounding-the depression 158' is preferably rendered light absorbent. As pointed out previously, optical contact maybe provided between thesurface fid-and the-tubefaceibyusingga layer of oil or Canada balsam-1 The mounting of the antihalation lens;-:5.-i: in.

the. bore 53 permits the antihalation'xlens tobe moved towardthe plane mirror surface 41 :torobs tain optimum optical conditions. If the member 44 is made of an optically suitable material such as glass or one of the suitable plastics pointed out above then the antihalation lens 5| maybe This What is claimed is:

1. A projection system for eliminating halation and correcting error in edge resolution introduced by said halation eliminating comprising a concave spherical mirror, a correcting plate positioned to transmit light refiectedfrom said spherical mirror, a projection tube for projecting light to said mirror, a viewing screen receiving light in the form of a readily perceptible image transmitted by said correcting plate, said tube .having a transparent end, the end being optically bounded by inner and outer concentric spherical surfaces, said inner and outer spherical surfaces having radii of such relative length that theli'ght produced upon excitation 'of any point on.v the luminescent screen is transmitted by the outer surface of the tube end withouttotal reflection, and a meniscus positioned in the optical path between said mirror and said correcting plate said meniscus being substantially, concentric with said spherical mirror.

2. A projection system comprising a concave spherical mirror, a correcting plate positioned to transmit light reflected from said spherical mir ror, a projection tube for projecting light to said mirror, a viewing screen receiving light in the form of a readily perceptible image transmitted by said correcting plate, an antihalation lens for said tube'optically bound by inner and outer concentric spherical surfaces, said inner and outer spherical surfaces having radii of such relative length that the light produced upon excitation of any point on the luminescent'tube screen is transmitted by the outer surface of the antihalation lens Withoutv total. reflection, and a meniscus positioned in theoptical path between said mirror and said correcting plate said-meniscus being substantially concentric with said spherical mirror, whereby to eliminate error in? troduced by said antihalationlens.

3. A projection system comprising a concave spherical mirror, a meniscus concentric with said concave spherical mirror positioned to transmit light receivedfrom said mirror,. a. cor-v recting plate positioned to transmit light received from said spherical mirror throughsaid meniscus, a. plane mirror, a projection tube-facing, said plane. mirror, said plane-mirror being-so located as toreflect light from said tube to-said spherical mirror, and an antihalation lens positioned before the face of said tube in thelightpath from the tube tdthe-plane mirror, whereby said meniscusconrects forerror introduced into the system by said antihalation lens.- I i e. AHprojection-system comprising a. concave spherical mirror, a-rneniscus positioned to: trans-- mit light received from said mirror, a correcting plate positioned to transmit light received from said spherical mirror through said meniscuasaid; meniscus-having meansto support a plane mi-rror, an aperture in. saidispherical mirror, a pro-@ jection tube facing said plane mirror through.

said aperture, saidplanemirror beingqso -locatedhas to reflect lightfrom said. :tube: to said.

spherical mirror, and an antihalation lenspositioned beforetthe face of said tube in the:light path. :from the tube-to the-.planemirror, meanswhereby said lens and. saidtube-u aremovable;

within saidhaperture toward and from said-plane mirrorfor adjustment, said meniscus being adapted to correct error in edge resolution-intro (Referenceson following page) 5 6 References Cited in the file of this patent FOREIGN PATENTS UNITED STATES PATENTS g' G C n D e e 3 reat Bri ain 0c a Number 'i 586,851 Great Britain Apr. 2, 1947 2,295,779 Epstem sept- 1942 5 t 13 253 Great Britain Feb 13 949 2,298,808 Ramberg Oct. 13, 1942 2,346,810 Young Apr. 18, 1944 OTHER REFERENCES 2,409,971 Bennett 22, 1946 Journal Scientific Instruments, v01. #22, Jan- 2,458,132 Baker Jan. 4, 1949 uary 1945, page 12 ,492,461 BouWe S Dec. 27, 1949 0 Telescoptics, Scientific American, August 1939, 2,517,774 Epstein Aug. 8, 1950 pages 118-123. 3,571,657 Bennett Oct. 16, 1951 Achievements in Optics, A. Bouwers, Elsevier 2,585,009 Henroteau F b- 1 1 5 Publishing Co., Inc., 1946, page 47. 

